1. Field of the Invention
The present invention relates to a pressure vessel having a pressure releasing mechanism which allows the pressure in the vessel to escape by itself when the internal pressure thereof has increased to such an extent as to blow off the lid of the vessel.
A pressure vessel mentioned herein refers to a vessel containing a quick lime which reacts exothermically with water when brought into contact therewith so as to heat a food or a pharmaceutical product separately accommodated in the vessel by the heat generated by the contact between the quick lime and water, or an aerosol vessel containing a cosmetic preparation and a liquefied gas.
Such a pressure vessel generally has a pressure releasing mechanism to prevent any explosion that would occur if an excessive exothermic reaction takes place in the former type of vessel, or if the internal pressure is excessively increased because the latter type of vessel has been thrown into a fire.
2. Description of the Prior Art
A conventional pressure releasing construction for a pressure vessel of this type has been disclosed in the specification of Canadian Patent No. 957,628.
In this pressure releasing construction, a metal upper lid b is double-seamed to a metal can body a of an aerosol vessel, as shown in FIGS. 10 and 11. A vertical lid plate portion c of the metal upper lid b which abuts against the inner wall of the can body has a large number of score lines d extending in the radial direction, each starting from halfway down the vertical lid plate portion c and extending over the top of the seaming. If the upper lid b is deformed from the state shown in FIG. 10 to the state shown in FIG. 11 by an increase in the internal pressure, the score lines d are broken so as to release the internal pressure therethrough, thereby preventing blow-off of the upper lid b. In such a construction, however, since the score lines d are provided over the seamed portion of the lid, they cannot be broken until the vertical lid plate portion c of the upper lid b and the top portion extending therefrom have been deformed to such an extent that they have risen. This means that there is the danger of a time lag between the excessive rise of the internal pressure and the escape of the pressure. In addition, it is illogical in terms of the function originally required of the internal pressure vessel to provide the score lines d in the portion of the lid which is to be firmly seamed to make the vessel completely air-tight.